Overview

Comprehensive Description

Description

A small green alga (up to 30 cm across) with a broad, crumpled frond that is tough, translucent and membranous. It is attached to rock via a small hold-fast .Ulva is sometimes eaten as "green laver", but it is considered inferior to purple laver.

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The sea lettuce is found at all levels of the intertidal, although in more northerly latitudes and in brackish habitats it is found in the shallow sublittoral. In very sheltered conditions, plants that have become detached from the substrate can continue to grow, forming extensive floating communities. The plant tolerates brackish conditions and can be found on suitable substrata in estuaries.

Contents

Ulva lactuca is a thin flat green alga growing from a discoid holdfast. The margin is somewhat ruffled and often torn. It may reach 18 centimetres (7.1 in) or more in length, though generally much less, and up to 30 centimetres (12 in) across.[1] The membrane is two cells thick, soft and translucent, and grows attached, without a stipe, to rocks or other algae by a small disc-shaped holdfast.[2]

Green to dark green in colour, this species in the Chlorophyta is formed of two layers of cells irregularly arranged, as seen in cross-section. The chloroplast is cup-shaped in some references but as a parietal plate in others[2] with one to three pyrenoids. There are other species of Ulva which are similar and not always easy to differentiate.

Ulva lactuca is very common on rocks and on other algae in the littoral and sublittoral on shores all around the British Isles,[3] the coast of France,[4] the Low Countries[4] and up to Denmark.[5] It is particularly prolific in areas where nutrients are abundant.[6] This has been the case off the coast of Brittany where a high level of nitrates, from the intensive farming there, washes out to sea.[7][8] The result is that large quantities of Ulva lactuca are washed up on beaches, where their decay produces methane, hydrogen sulphide, and other gases.[7][9]

Certain environmental conditions can lead to the algae spreading over large areas. In August 2009, unprecedented levels of the algae washed up on the beaches of Brittany, France,[10][11] causing a major public health scare as it decomposed. The rotting thalli produced large quantities of hydrogen sulfide, a toxic gas which, like hydrogen cyanide, inhibits cytochrome c oxidase, inhibiting cellular respiration and resulting in critical cellular hypoxia. In one incident near Saint-Michel-en-Grève, a horse rider lost consciousness and his horse died after breathing the seaweed fumes. Environmentalists blamed the phenomenon on excessive use of fertilizers and the excretion of nitrates by pig and poultry farmers.[10] In a separate incident at the same beach, a truck driver and several schoolchildren died after taking part in the cleanup without protection.[citation needed]

The sporangial and gametangial thalli are morphologically alike. The diploid adult plant produces haploidzoospores by meiosis, these settle and grow to form haploid male and female plants similar to the diploid plants. When these haploid plants release gametes they unite to produce the zygote which germinates, and grows to produce the diploid plant.[12][13][14]